There are conventional clamps for a disk rotational driving device as shown in FIG. 7 disclosed in Patent Literatures 1 and 2.
Such a clamp 101 has a central part 103, a centering hole 105 formed through the central part 103, a peripheral part, and a contact portion 107 formed along the peripheral part. The clamp 101 is attached to a hub 109 to be rotary driven of a hard disk drive. A clamping screw 111 is screwed into the hub 109 so that a fastening head 113 of the clamping screw 111 fastens the central part 103 to the hub 109.
The centering hole 105 of the clamp 101 is fitted to a centering projection 115 circumferentially formed on the hub 109, to center the clamp 101.
The contact portion 107 of the clamp 101 fixed to the hub 109 comes in contact with a magnetic disk 117 attached to the hub 109, to apply pressing load onto the magnetic disk 117 in an axial direction, to thereby fix the magnetic disk 117 to the hub 109.
The centering hole 105 of the clamp 101 has a centering surface that fits the centering projection 115 of the hub 109. According to the related arts, the axial length of the centering surface is equal to the thickness of the clamp 101, and therefore, is insufficient to secure a centering surface.
Namely, a guiding function of the centering hole 105 with respect to the centering projection 115 is insufficient when attaching the clamp 101 to the hub 109. This results in deteriorating a fitting efficiency and a fitting accuracy of the clamp 101 with respect to the hub 109.
The insufficient guiding function may cause an assembling error, e.g. placing the centering hole 105 on the centering projection 115.
On the other hand, the clamp may be provided with a centering cylindrical portion that is formed by burring to axially protrude. An inner circumference of the centering cylindrical portion defines a centering hole. This configuration may elongate the axial length of the centering hole and improve the guiding function.
The part of the clamp bent by burring, however, has a rounded section whose radius is equal to the thickness of the clamp. This results in reducing a fastening surface on the central part 103 of the clamp 101 and deteriorating a fastening force applied by the fastening head 113 of the clamping screw 111.    Patent Literature 1: Japanese Unexamined Patent Application Publications No. 2003-338101    Patent Literature 2: Japanese Unexamined Patent Application Publications No. 2004-95054